151 (dimethyl gloiosiphone A)

Scheme 28 Synthesis of methyllinderone [119] and dimethyl gloiosiphone A [120,121]

It is worth noting here the remarkable reactivity of the unsaturated acyl radical 152a. The Baldwin rule predicts that 5-endo cyclization is not favored. However, actually, this mode (131 ^ 132) was found to be advantageous, and no product was obtained from essentially favored 5-exo cyclization (131 ^ 132exo). According to several calculations, the net cyclization is best explained by non-radical ring closure from ketene-substituted a-carbonyl radical 152b to the cyclized radical 153 (i.e., nucleophilic attack of OH on C = C = O with a dipolar n-radical-stabilized transition structure 154TS) [124]. Independently, the similar chemical behavior of the ketenyl radical was reported by Pattenden [125,126] (Scheme 29).

Additional reactions involving the electron-deficient oxygen center were carried out by employing a hypervalent iodine reagent, because the facile displacement on iodine with nucleophiles (e.g., NH2 and OH) and the superleaving ability of newly formed iodine intermediates endows the electron-deficient center of these heteroatoms. Whereas this type of rearrangement has already been found for nitrogen, the case for oxygen was provided for the first time by the reaction of 4-hydroxy-2-cyclobutenone to 2-(5H)-furanone [127]. PhI(OAc)2 is the reagent of choice, and a better result was attained in reflux-ing methanol to give the 5-methoxy-2-(5H)-furanone 158; Scheme 30 illus-

EtO OE1 EtQ .OEt


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